1. Field of the Invention
The present invention relates in general to a control apparatus for an a.c. motor driven through frequency converters and in particular to a control system which allows the a.c. motor to be operated at a high speed even when the upper limit of the output frequency from the frequency converter is set at a low value.
2. Description of the Prior Art
As is well known in the art, the a.c. motor driven through the frequency converter composed of thyristors is a motor capable of being driven at variable speeds and having no commutator to make the maintenance therefor to be facilitated. This type of motor is often referred to as the commutatorless motor.
It is also well known that the rotational speed of such commutatorless motor depends on the output frequency of the frequency converter, as can be expressed by the following equation: EQU N = 120 f/p (1)
where
N: rotational speed of motor
F: OUTPUT FREQUENCY OF THE FREQUENCY CONVERTER, AND
P: NUMBER OF POLES OF MOTOR.
On the other hand, an upper limit is imposed on the output frequency of the frequency converter. In particular, the upper limit of the output frequency is relatively low in the case of a frequency converter in which commutations of thyristors constituting the converter are effected by utilizing an a.c. source voltage. For example, in a cyclo converter which is supplied with power from an a.c. power source of a commercial frequency to control firing angle of the thyristors for producing a sine wave voltage having a variable frequency, the upper limit of the output frequency is usually on the order of one third of the a.c. source frequency. The permissible highest speed of the motor is restricted by the above equation (1). The motor can not be driven at a high speed exceeding the permissible limit, even when load apparatus requires the high speed.
Attempts to solve the above problem have been already proposed. For example, in the article "Cyclo converter Control of the Doubly Fed Induction Motor" by WILLIS F. LONG published in "THE INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS", VOL. IGA-7, No. 1 (1971), pp. 95-100, there is disclosed an induction motor control system in which the primary and the secondary windings of the motor are connected in series is opposite phase relation relative to each other and excited by a common cyclo converter, thereby to attain the rotational frequency as twice as higher than the frequency of the exciting current. However, the induction motor exhibits the series winding motor characteristics which are disadvantageous in certain application.
For example, it may be contemplated that the commutatorless motor is to be used for driving a rolling mill for steel materials. In such case, the shunt motor characteristic is required for the driving motor, since the series winding motor characteristic is incompatible with load characteristic.
Furthermore, a large capacity is required for the slip ring which serves to connect the primary and the secondary windings to each other. In other words, electric power supplied to the primary winding and the secondary winding is determined by the excitation frequency. Thus, in the case of the induction motor disclosed in the above literature, electric power supplied to the secondary winding is a half of the rated capacity of the motor. As the capacity of the motor is increased, the capacity of the slip ring is also correspondingly increased. At present, it is considered that the upper limit of the slip ring capacity should be on the order of 1000 KW from the economical viewpoint. Consequently, the motor of a large capacity is usually regarded impracticable.